Wave transmission



y 29, 1952 A. SAMUEL 2,605,323

WAVE TRANSMISSION Filed Aug. 51, 1946 3 Sheets-Sheet 1 FIG.

I CRYSTAL RM. osc- 1 I z CRYSTAL m FIG. 2A

FIG. 2B

FIG. 3

7'0 HOP. DEFL. VANES 0F TO A l/ERZ' DEFL.

vmvss OF CPYS TALS 8 w T s c 2 V Z D/RECT/ONAL COUPLE/P f, /o

/N 5 N 7' 0/? B ,4. L. SAMUEL ATTO NEV July 29, 1 A. L. SAMUEL WAVE TRANSMISSION Filed Aug. 51, 1946 3 Sheets-Sheet 2 TO VERZ' DEE VANES OF C.R.O.

FIG. 4

A T TE NUA TOR.

Dl/PEC T/ONAL COUPLE/P /RES. STRIP \RES. STRIP (RES. ST/P/P INVENTOR By A.L. SAMUEL ATTORNEY A. L. SAMUEL WAVE TRANSMISSION July 29, 1952 Filed Aug. 51, 1946 3 Sheets-Sheet 3 DIRECTIONAL DIPECT/ONAL COUPLE/P C O UPLE I? FIG. 7

. COUPLER r P w" v.

DIRECTIONAL COUPLE/P INVENTOR AL. SAMUEL ATTO NEV Patented July 29, 1952 2,605,323 I WAVE TRANSMISSION I Arthur L. Samuel, Champaign, Ill., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of NewYork Application August 31, 1946, Serial No. 694,343

6 Claims.

spacing of crystal detectors in a wave guide fed by constant amplitude, frequency-modulated waves.

Another feature of the invention is a pairing of four such spaced crystals into two pairs, each pair being characterized by a spacing and a poling in opposition between member crystals.

Another feature of the invention is a frequency modulated source of constant amplitude connected to a main wave guide terminated in the unknown impedance.

Another feature is the use of sampling directional couplers connected to the main wave guide for reducing the ratio of reflected to incident waves wherebycrystal-law errors may be minimized.

Another feature of the invention is a terminated wave guide fed by input waves of constant amplitude over an extended frequency range, and the derivation by detection therefrom of two separate voltages proportional to Vr cos and VT sin 0 for application to the horizontal and vertical deflecting vanes respectively of an oscilloscope, where Vr represents the amplitude of waves reflected by the termination and 0 their phase angle relative to the incident waves.

An experimental determination at ultra-high frequencies of the input impedance as a function of frequency for any proposed circuit element requires measurements of its complex impedance at a number of different frequencies over a prescribed frequency range. A common method therefor involves, (1) observations of the standing wave along a uniform transmission line terminated by the unknown impedance, (2) the computation from these observations and data of the input impedance or of the reflection coeflicient at a number of different frequencies and (3) the presentation of these data on a transmission line chart, usually of the reflection-coefficient-plane type.

Ihe oscillographic method, which is to be described, oifers a convenient and rapidmethod of presenting the data directly and visually in the desired form without computation. The principal advantage of this method over the conventional point-by-point method is one of speed and convenience. Also, it is possible with the oscillographic method to observe the variations in the input impedance of devices under transient conditions, for example, such as are produced by rotating joints or by random variations in the input impedance of an antenna produced by reflections.

In accordance with the invention, applicant provides an apparatus and method for oscillographically presenting the reflection coefficient both in amplitude and phase over the ultrahigh frequency and microwave range. A frequency modulated source provides constant amplitude input waves Vi over an extended frequency range for a Wave guide or coaxial transmission line either terminated in an impedance to be measured or having an inserted four-terminal circuit element therein, Whose transmission-frequency characteristics are to be determined. Four detecting crystals, are arranged along the guide and paired in couples in such a way as to mix the incident wave Vi and reflected wave Vr to derive thereby separate voltages proportional to Vr sin 0 and VT cos 6 where 0 represents the phase angle of Vr, for application to the horizontal and vertical deflecting 'plates respectively of the oscilloscope.

The oscilloscope electron beam thereby traces on the screen in polar coordinates r, 6 curves or patterns which represent the frequency variation of the reflection coefficient in amplitude and phase.

The arrangement of crystals for providing defleeting voltages for the vanes or plates of the oscilloscope, involves a spacing of the crystalsalong the guide and a pairing of the crystals spaced from each other. The outputs of the respective crystals of a pair are poled subtractively or in opposition to leave a resultant voltage of the form ViVr cos 0 and'VZ-VT sin 0.

ing a linear type of operation for the crystal 7 mixers.

Fig. 1 shows a schematic of the impedance viewer circuit;

Figs. 2A, 2B show typical oscilloscope patterns obtained therewith Fig. 3 illustrates a modification utilizing directional couplers;

Fig. 4 illustrates another modification using a single directional coupler and an attenuator;

Fig. 5 illustrates a modification using hybrid Ts and directional couplers;

Figs. 6 and 7 show modified circuits for determining transmission characteristics.

A simple apparatus for supplying the oscilloscope with the deflecting voltages VT cos 0 and VT sin 0 is shown schematically in Fig. 1.

A frequency modulated oscillator l supplies input waves V1 of constant amplitude over a frequency band centered in the ultra-high frequency or microwave range to a. transmission line 2, terminated by an impedance 3 to be measured. The impedance 3 which may, for example, be a tube, a resonant cavity, or some substance endowed with resistive and reactive properties, may set up standing waves in the transmission line. The amplitude of the reflected wave will be represented by Vr.

The frequency modulated oscillator I is of the magnetic focussed, velocity modulated type, characterized by two spaced resonant cavities as disclosed in the United States application of A. L. Samuel, Serial No. 441,937, filed May 6, 1942, now Patent No. 2,410,840, and using a small diameter feedback section. Frequency modulation is accomplished by a small silver vane which is rotated in the one cavity at 1800 revolutions per minute by a synchronous motor. The cavity oscillates in its second TE mode. The tube is at one voltage maximum and the rotating vane is at the other. The vane varies the capacity at the second voltage maximum.

A permanent magnet is used in the focussing of the electron beam. This oscillator has a frequency sweep of 500 megacycles centered about 4200 megacycles.

It should be understood that it may be possible to use other forms of frequency modulated oscillators using tubes of the reflex, single cavity or other types.

Four probes andcorresponding crystals X1, X2, X3 and X4 are used to sample the waves existing in the transmission line 2, which may be either a wave guide as shown or a coaxial line. These pick-up probes are connected in pairs, the two probes of each pair being spaced along the line by a quarter wavelength while the two pairs are'staggered by an eighth wavelength where A is the wavelength in the guide. The output from each probe goes directly to a corresponding crystal detector. The two crystals X1, X2 of one pair of probes are balanced, the difference in their outputs being impressed on one pair of deflection vanes, i. e., the horizontal deflection vanes in the oscilloscope. Similarly, the difference in the outputsfrom the second pair X3, X4 is impressed in a like manner on the other pair of deflecting vanes, i. e., the vertical deflection vanes in the oscilloscope. If then, the input I to the transmission line 2 is varied in frequency but not in amplitude, and if the crystals follow a square law of operation, the oscilloscope spot will trace a path representing the desired curve on the reflection coeflilcient plane. The center of the reflection coefficient plane is located by interrupting the output of the driving oscillator I at the end of each frequency excursion.

Care must be taken to insure that the probes are not large enough to distort the fields in the wave guide or coaxial line seriously enough to roduce interaction between probes. The connecting of the probes in pairs, spaced apart At crystal (X2) the incidentwave V1 will bedelayed by 1r/2 radians and the reflected wave Vr will be advanced by w/2r'adians, corresponding to the time required by the waves to traverse the quarter wavelength section of transmission line separating the two probes. The output from crystal (X2) will then be .2 v.-v. cos a+-R. F. terms 2 The difference in the outputs which appears between the horizontal defiection or abscissa vanes of the oscilloscope is At crystal in, the incident wave will'b'e delayed by while the reflected wave will'be advanced by corresponding to the time required by the waves to-traverse the ooiy spacing between" pairs. v

Crystal (X3) will have an output of [V,-' cos (wt1r/4)+V, cos (wt-|1r/4+0)] V i! V 2 7 2 V.-V, sin 0+R. Eterms (3) v.- cos rti-ar rwv. cos aware- 1 f +v.- sin +1 F; terms 4 the value of Vr in magnitude and phase, and

present a curve or pattern on the screen as shown by Way of example, in Figs. 2A and 2B. The proper scale factor for the oscilloscope and its accompanying amplifier can readily be obtained by terminating the-transmission line in a short circuit so that (VT) (Vi) and calling the accompanying deflection unity. Since bydefinition Vr for unity V1 is the reflection coeflicient, the desired relationship has been verified.

- Some exemplary patterns or curves obtained with'the-impedance viewer circuit of Fig. 1 are illustratedin Figs-2A and 2B. Fig. 2A shows the oscilloscope pattern when a longtransmis sion line is terminated by a short circuit. This should, of course; be an arc of the-unit circle, the length of the arc depending upon the length of the transmission line and upon the frequency excursion. The departure from a true circle is 'a'measure of the refinement of construction in some components of the system. ,The irregularities at the endsof the circular arcinkthe picture are due to variations in input power level and a reversal in the direction of the fre- Likewise. crystal (X4) willhave'an output of I quency sweep during the switching period'which are associated with the transient behavior of the power supply and switching circuits (not shown). Fig. 2B shows the input impedance Z of an over-coupled double-tuned circuit adjusted to match the transmission line at two different frequencies. I

- The reflection coefficient plane has been found to be a desirable medium for presentingimpedance data. As is now generally known, the Smith chart which is disclosed in Electronics, January 1939, Transmission line calculator by P. H. Smith is obtained by a bilinear transformation to the reflection coefiicient plane of the coordinate system on the impedance plane- Since the reflection coeficient is always less than unity (that is, for passive'circuits) the plane is bounded by a unit circle, the reflection coefficient p being given by the vector distance from the origin .to any point on the plane. The impedance coordinates are transformed into orthogonal families of circles and the bilinear nature of the transformation requires that circles remain circles and that angles be preserved. Distance along a lossless transmission line appears on the chart as distance along the circumference of a circle-coaxial with the center of the chart, the absolute magnitude of the reflection coefficient remains constant under these conditions. Sincetranslation by a quarter wavelength along a transmission line results in an inversion of the impedances with respect to the line characteristic impedance, and since this corresponds to a rotation of Ir radians on the reflection'coeflicient plane, a transformation between impedan'cesand admittances requires only that the reference-axis be rotated by 1r. Impedances plotted on the reflection plane can therefore be readily-trans' formed into admittan'ces and back ancesas theoccasion demands.

Crystal-law errors can be substantially eliminated by anyarrangement which attenuates the reflected wave component V1: so that it is always small compared with the'direct wave component V1. The action of the crystals in this case is analogous to a converter. The direct wave corresponds to the beating oscillator wave in the conventional converter and the reflected wave takes the place of the signal. This'results-in alinear signal response characteristic. It does notv modify the requirements that the input power level be constant since the magnitude of the reflected wave will still vary with the magnitude of the incident wave. 9A variety of different circuits are disclosed hereafter which permit the separation of the incident wave and reflected wave components so that one can be attenuated relative to the other. Such circuits are shown in Figs. 3 to 5, in terms of wave guide structures although they are equally well adapted to coaxial line systems.

In the circuit of Fig. 3, two directionaljcow plers I 0, I I connected to the main waveguide to imped- 2 are used to sample the incident and reflected waves. The coupler II which samples the reflected wave set up by Z,-the impedance to be measured may be constructed. to have a greater coupling attenuation than the coupling attenuationof the coupler l0 sampling the incident wave; The probes and crystals X1, X2, 11%, X4,

guide 8, are positioned absorbing terminations" T for dissipating undesired wave componen The crystals X1, X2, X3, X4 of Figs. 1, 3, etc., may be silicon, germanium, or any othertype normally used for detection and conversion at ultra-high frequencies and microwaves.

The crystal outputs from X1Xz, and X3X4 in Fig. 3 are shown fed into amplifiers A, A respectively, which preferably are balanced with a large amount of negative feedback to any unbalanced components. 1 1

Provision must be made to adjust the gain of th two amplifiers to equality. Variations in crystal sensitivity can be compensated by varying the probe length or by means 'of' potentiometers. However, it is not sufficient to adjust the crystals to the same sensitivity. at a single level only. If the crystals do not follow the samelaw, the cancellation of the mathematical terms in the above analysis will not occur. In the equipment used to obtain the curves of Figs. 2A and 2B no effort was made to select crystals which matched and the curves are therefore not exact. Thistype of error is reduced by the same expedient which eliminates the effects of non-square law operation of the crystals. I v

of the type 7 level over-the band.

7 It should also be noted that care must be ex'er cisedin the design of the crystalcircuits to make them sumciently broad 'in their :i-requenjcy.,;re sponse-characteristi-cso that theireoutputs' remain It is; of course, possible to 10116111180- tional coupler as; shown "by Eig.; -4;; The use ofa resistive attenuator 9 for reducing theratio of Vi to V1 then becomes mandatoryifanimprovernent in linearity isto be, obtained. Iii-"general, the use of a single directional coupler as in Fig. 4 to sample both-wave components imposesscvere requirements on impedance matching.

In operation, the incident wave Vi from the frequency modulation oscillator is sampled by the directional coupler and propagated along the arm I4 of the auxiliary wave guide 3. The reflected wave V1 due to 'the'impedance Z is sampled by the directional couplerand propagated into arm. 15, whereits level with respectto V1 may be reduced sufficiently by attenuator 9 'with impedance matching, tapered terminals, to substantially eliminate the effects-of crystal-law deviations.

The use of directional couplers to sample the waves 'Vi, V1 also makes itpossible to dispense with the use of probes as shown in Figs. 1, 3 andto use hybrid T junctions to obtain the necessary additions and-subtractions. Such a circuit using wave guides is shown in Fig. 5, and is characterized by broad-band frequency characteristics.

Two inputdirectional couplers .20, 2| and two output directional couplers 22,23 are'shown, with the sampling of incident and reflected wavesrepresented-by arrows as shown.

A sampled incident wave isapplied from. cou pler 20 to the hybrid T 25 at one end anda sampled, reflected wave from couplerZZ is appliediat the Opposite end. Additions and subtractions take place in crystals Xixa'connected'to the conjugates Band H branches respectiyely of'the hybridT25. l I

Asimilar action takes place with respectto incident and reflected wave samplings in the. sec-- ond hybrid T 26, which is displaced I v bi from hybrid'T 25-t'o provide the necessary phase difference, V

The hybrid Te 25, 2S are-of the type disclosed in the United States application of W. A. Ty-rrell, $eria1-;No;158 l,235 filed March 6, 19 45,now- Patent N0.-2,445,896 and provide the substantial equivaleiitofa- V probe spacing :in the electric (E) and magnetic (H) arms thereof. e

The balanced crystal outputs are then routed to the oscillograph amplifiers as previously described. .1 r 1 s In Fig. the unknown impedance- Z is connected to an end section l2, bent up perpendicularly to the main waveguide 2. r

The resistance strip' terminations, which are locatedat the ends of the; auxiliary guides 3', 3' eliminate by absorption undesired waves.

The crystals, X1X2X3X4 in Fig. 5. are designed for broad-band operation, and the pick-ups associated therewith have. enlarged knobs or heads.

The sensitivity of the'directional coupler and hybrid junction system (Fig. 5) can be made somewhat greater than that of the simple probe method since it is possible to obtain much larger samples of the incident wave by these means; The method suffers from the fact that greater lengths of line are involved and this tends to limit the frequency range over which reasonable accuracy can be obtained. As satisfactory hybrid T junctions are less easily constructed in coaxial line systems, the construction in Fig. 5 is deemed more practical in wave guide systems.

The circuits illustrated in Figs. 6 and 7 are intended- ;to measure transmission-frequency characteristics of four-terminal networks interposed-in a transmission line or wave guide, and to present the transfer impedance (or admit.- tance) oscillographically in amplitude and phase. Specifically, in the circuit of Fig. 6, the incident wave-Vi is sampled by means. of an' input directionalcoupler 3.] and combined with a sample of the transmitted wave VT, derived from the output of network Z and. propagated into guide section 34. The outputdirectional coupler 32 is located on the main wave guide beyond the network Z. The additions and subtractions by means of crystals XIX2X3X4 and the probes occur in a manner similar to that described previously for .Fig. 1. The terminal strips T are absorberssi-miglar in structure and function to the resistive strips of Fig.5. The circuit of Fig. 7 is analogous in structure to that shown in Fig. 5, using input directional couplers lfl, l I, output directional couplers 52, 53, and hybridTs 55-, 56 to measure oscillographically the transmission properties of a four-terminal network both in amplitude and phase.

What is claimed is: r 1. Apparatus for measuring the reflectioncoefficient of an impedance-terminated waveguiding passage from the direct and reflected waves therein comprisingin combinatiom means for impressing ultra-high frequency constant amplitudewaves upon said passage, directional coupler means coupled to said passage for deriving oppositely directed. waves outside said passage proportional tosaid direct and reflected waves, respectively, means for deriving from said :derived oppositely directed waves a first wave same .ple and a secondwavesample displaced degrees in phase therefrom, 'whereby sa-idl samples correspond in relative amplitude and phase with the waves at two points in said. passage spaced substantially apart, whereX -is the wavelengthin the guide, means for deriving third and fourth wave samples displaced 90 degrees in phase apart and correspondinglikewise in relative amplitude and phase with said waves at two other points in saidpassage, one of said points being spaced midway of the firstsaid two points and the other a H from said one point, meansior rectitying each ofsaid wave samples, means for differentially combining the rectifiedsaid first and second wave samples to obtain a voltage proportional to VT cos 0 and said third and fourth-wave-sam-ples toobtaina voltage; proportional to Vr sin 0-,'where Vi represents the amplitude-of the reflected wave and 0 its phase angle relative to the direct wave.

2. Apparatus for measuring therefiection coefficient of an impedance-terminated main wave guiding 1 passageirom 'thedirect and reflected waves therein comprising in combination, an ultra-high frequency source of constant amplitude waves connected to said main wave guiding passage, directional coupler means comprising auxiliary wave-guide means having absorbing impedance terminations, said directional coupler means being coupled to said main passage for deriving superposed oppositely directed waves in said auxiliary wave-guide means proportional to said direct and reflected waves respectively, said directional coupler means having, different directionally-selective coupling attenuations for the said direct and reflected waves, means for deriving from said waves in said auxiliary wave-guide means a first pair of wave samples corresponding in relative amplitude and phase with said waves at points in said main passage spaced substantially v s g s 7 Q 4 apart, where A is the wavelength in the guide, means for deriving from said waves in said auxiliary wave-guide means a second pair of wave samples corresponding likewise in relative amplitude and phase with said waves at two other points in said main passage, one of said points being spaced midway of the first said two points "and the other a. i

, v 4 I v from said one point, means for rectifying each of said wave samples, means for differentially combining the said first pair of wave samples to obtain a voltage proportional to V1 cos and the said second pair of wave samples to obtain a voltage proportional to Vr sin 0, where VT represents the amplitude and 0 the phase angle of said reflected wave relative to said direct wave.

3. An impedance viewer for representing the reflection coefficient of an impedance-terminated main wave guiding passage from the direct and reflected waves therein comprising in combination, means for impressing frequency-modulated constant amplitude waves extending over a limited frequency range upon said main passage, directional coupler means comprising an auxiliary wave-guide passage having absorbing impedance terminations at each of its ends, said directional coupler means having directionallyselective aperture couplings to said main passage for deriving superposed oppositely directed waves in said auxiliary passage proportional to said direct and reflected waves respectively, said directional coupler means having different directionally-selective coupling attenuations for the said direct and reflected waves, four coupling probes connected to said auxiliary passage at successive points separated by approximately in the guide at the frequency of said impressedficient, a cathode-ray oscilloscope including means for deflecting the cathode ray in mutually perpendicular directions, and means for applying each of said voltages to a respective one of said deflecting means to present said reflection'coefficient as a polar diagram.

4. Apparatus for measuring the reflection co.- efiicient of an impedance-terminated main wave guiding passage from the direct and reflected waves therein comprising in combination, means for impressing ultra-high frequency constant amplitude waves upon said main passage, directional coupler means comprising a pair of auxiliary wave-guide passages on opposite sides of said main passage, said auxiliary passages having resistive terminations at each of their respective ends, said directional coupler means .being of said waves impressed upon said main passage,

Where k is the wavelength in the guide, mixing crystal detectors inserted in the conjugate arms of each of said hybrid Ts, means for differentially connecting the said detectors of respective hybrid Ts in pairs to obtain a first voltage proportional to VT cos ofrom one of said hybrid Ts and a second voltage proportional to V1 sin 0 from the other of said hybrid Ts, where VT represents the amplitude and 0 the phase angle of said reflected wave relative to said direct wave.

5. An impedance viewer for representing the reflection coefiicient of an impedance-terminated main Wave guiding passage from the direct and reflected waves therein comprising in combination, means for impressing frequency-modulated constant amplitude waves extending over a limited frequency range upon said main passage, a pair of directional couplers comprising a pair of auxiliary wave-guide passages on opposite sides of said main passage, said auxiliary passages having resistive terminations at each of their respective ends, each of said directional couplers having a first directionally-selective aperture coupling to said main passage at one point and, at a second point spaced from said first point, a second directionally-selective aperture coupling to said main passage for deriving oppositely directed waves in each of said auxiliary passages proportional to said direct and reflected waves respectively, said directional couplers having different directionally-selective couplingv attenuations for the said direct and reflected waves, a pair of wave-guide hybrid Ts each having their collinear arms in respective ones of said auxiliary passages, said hybrid Ts being displaced relative to each other by approximately at the frequency of said waves impressed upon said main passage, where )\g is the Wavelength in the guide,.mixing crystal detectors inserted in the conjugate arms of each of said hybrid Ts, means for differentially connecting: the said detect'orsaof respective hybrid Ts in pairs, to obtain: two septarate voltages related 'in 1 amplitude and: phasemosa'i'd reflectionfcoefficient; arcathodeeray OSOiHO-I scope including means for deflectingitheicathode ray in mutually perpendicular directions,;'and means for applying' each of 'said" voltages to a respective one of s'aid'deflecting means to present said refiection coefficientas a polar diagram.

'6; A nimpedanc'e Viewer for representing. the reflection coefiici'ent of an impedance-terminated main wave" guiding passage from the direct and reflected wave's therein comprising in combination, means for impressing frequency-modulated. constant amplitude \vaves extending over alimited frequency range upon said main passage, a pair of directional couplers comprising a pairof auiciliar-y Wave-guide passages on opposite sides ofsaid mainpassage, each of said 'directional couplers having a first directionally-selective aperture coupling to said mainpassage at one pointandi at a second point spaced from said first-point'a second; directionally-selective aperture coupling to said main passage for deriving oppositely directedwaves in each of said auxi1-. iary passages proportional to said direct and reflected Waves respectively, a-pair of wave-guide hybrid Ts each having their collinear arms in respective ones of said auxiliary passages, said hybrid Tsbeing. displaced relative, to each other b a p bx ma v l at thefrequency or said waves impre'ssed upon said 'm'ain pa'ss'agegWlir-Ag i's -th'e wavelength in the guide; mixing: detectorsinserted ih the conjugat'ei arms of each of s'aid hybrid 'Ts means for differentially connecting the said detectors "of respective *hyb'rid Tfsin pairs to obtain two separate voltages related in amplitud'e and phase t'o said reflection -coefficierit," means lfoifcoinbining said two voltages i'IYSpace- 'Quadrature', and fmeans for indicating th'e vectorsum of} said quadrature voltagesto present sai'di-refle ctibn coefficient las a polar diagram.- i I 1 REFERENCES onian l The, following references, are record in the file of this patent: UNITED STATES PATENTS Number Name" Date 2,337,934 V Scheldorf Dec. 28, 1943 2,40%,797 'H'a'nsenm; 4 Ju1yL30,i1946f 

